Emissions control

ABSTRACT

A cost-effective solution to removing particulates from diesel, especially light-duty diesel engines incorporates an oxidation catalyst ( 2 ) effective to convert NO in the exhaust from the engine to NO 2  and a particulate trap ( 4 ) which traps no more than 85% by weight of the particulate, optionally by permitting gas to by-pass the trap.

This application is a U.S. National Phase application of PCTInternational Application No. PCT/GB99/00626.

The present invention concerns improvements in emission control, andmore especially it concerns the control of emissions from dieselengines.

Diesel engines fall into two main categories, namely heavy duty, beingprincipally large engines for trucks, buses and prime mover vehicleengines, ships and boats and stationary engines, and light duty engines,used in smaller trucks and cars. With the increasingly demandingregulation of emissions from all sorts of engines, attention is nowbeing paid to control of emissions such as particulates and NOx fromdiesel engines. We have, in EP 341,832, described a device marketed asthe Continuously Regenerating Trap (“CRT™”) by Johnson Matthey. Thisdevice traps sooty particulates and causes their combustion by exposingthem to NO₂ generally generated by catalytically oxidising NO present inthe exhaust stream. This device has met with considerable success incontrolling particulate emissions from heavy duty diesel engines, andcan achieve zero emissions of particulate. That is, the CRT as marketedis approximately 100% effective to remove particulates (as defined inthe regulations).

We have realised that a conventional CRT may not be cost-effective tocontrol emissions to European Stage III, IV, or higher, regulations fromlight duty engines. A number of different options for controllingparticulates and NOx are available, and engine manufacturers havehitherto favoured engine design and management solutions. In general,engine design itself can satisfy European Stage II emissions standardsas regards NOx and particulates, but the characteristics of the dieselengine are such that engine design cannot improve upon about 0.4 g/Kmparticulates without a serious increase in NOx, which is difficult todeal with under lean conditions. There still remains, therefore, a needfor effective systems to meet these emission regulations whilstincreasing engine design options.

EP 0758713 (Toyota) describes an addition to the CRT principles,involving the absorption in a NO_(x) absorbent of NO from gases leavingthe CRT-type combination of oxidising catalyst diesel particulate filteror trap, formed by one of the reactions of NO₂ with carbon particles.

The present invention provides an emission control exhaust gasaftertreatment apparatus for diesel engines comprising a source of NO₂,especially an oxidation catalyst effective to convert a portion of NO inthe exhaust gas from said diesel engine into NO₂, and a particulate trapcharacterised in that an exhaust gas by-pass is provided so that aportion of the exhaust gases do not pass through the trap, such that atmost 85% by weight of total engine-out particulates are collected andcombusted in the presence of said NO₂ in said trap. According to variousembodiments of the present invention, the quantity of particulatescollected and combusted may be at most 85%, 60% or at least 50% byweight. It is, however, important that all particulates collected arecombusted (over several operating cycles, but not necessarily over asingle operating cycle), in order to avoid accumulation and blocking ofthe filter. The presence of the by-pass alleviates build-up of backpressure with consequent degradation of engine performance.

The by-pass may permit a portion of the exhaust gases to escape thetrapping means either continuously or when substantial trapping ofunburnt particulates has occurred. Desirably, the trapping means isdesigned to be “fail-safe”, that is whilst it is effective to capture 50wt % or more of sooty particulates under normal conditions, the trappingmeans decreases its collecting efficiency whether by exhaust gasesby-passing the trapping means or otherwise, if the collecting meansbecomes “saturated” or clogged up. This concept is, we believe, new indiesel exhaust treatment systems.

The invention accordingly provides a method of controlling emissions,especially particulate matter, from diesel engine exhaust gases bytrapping and subsequently combusting said particulate matter, comprisingtrapping at most 85 wt % of particulate matter in the presence of NO₂,and causing a portion of said exhaust gases to by-pass said particulatetrapping means.

The source of NO₂ is especially conveniently an oxidation catalyst ofgenerally known type, capable of converting at least a portion of the NOcontained in diesel exhaust gases into NO₂. However, the presentinvention includes within its scope variations including other sourcesof NO₂. Such sources could include compressed NO₂, other chemicalsources which decompose or react, possibly over a catalyst, to yieldNO₂, such as nitric acid, ammonia, urea, etc. The invention includeswithin its scope all ways of making NO₂ from the components of theexhaust gases. These may include: adding one or more catalyticcomponents to the fuel, so that the components deposit on exhaustcomponents including the filter; catalysing the filter or parts thereofwith catalysts active at low temperatures to convert NO to NO₂;utilising a plasma to generate NO₂ by treatment of all or a portion ofthe exhaust gases, and other methods available to the skilled person.

In the invention, the particulate trap, or trapping means, is designedto be less than 100% effective, and desirably this brings severaladvantages, the first of which is considerably reduced back pressure.Light duty diesels are very much less capable of coping with backpressure from exhaust gas systems than heavy duty diesels, because theytend to be of smaller capacity and power, and in extreme cases theengine can be ruined.

The present invention is also intended to cater for some of the problemsthat can arise in practical usage of light duty diesels. If such enginesare used in small cars or vans which are used at low speeds in towns forlarge portions of their operations, the exhaust gas temperature tends tobe quite low, perhaps not more than 100-120° C. Under these conditions,although sooty particulates are generated in less quantities than underheavy load, the temperature is below that for the most efficientoxidation of NO to NO₂ and hence there can be insufficient NO₂, or thereaction temperature is too low for effective combustion of the sootyparticles. Accordingly, for many vehicles for much of the time, thetrapping means should collect sufficient of the particles to meet theemission regulations, but using a design that collects such particulatesfor subsequent combustion when conditions are improved, and permits theexhaust gas to pass through without excessive back pressure. In suchsystems, at least 50 wt % of particulate matter is trapped andsubsequently combusted when operating conditions in the same orsubsequent operating cycles are improved.

It will be recognised that the prime purpose of the invention is toremove a proportion only of the particulates from the exhaust gasstream. This is intended to be adequate to meet the appropriate emissionregulation when combined with engine design and management improvements.The reduced efficiency in collection of particulates of the presentinvention brings about a significant reduction of cost, however, from areduced volume and weight for both the catalyst and the trap compared tothe state of the art CRT designed for such an engine.

The oxidation catalyst may be any that is effective to convertsufficient NO to NO₂, and is suitably a high platinum loading catalystcarried on a ceramic or metal honeycomb catalyst support. It isenvisaged that in addition to reducing volume and weight of thecatalyst, savings may be made in precious metal loading, thus reducingthe cost yet further.

The particulate trap may suitably be a woven or knitted wire mesh orperforated metal or a suitable ceramic material. Preferred traps includeespecially those known as wall flow filters. The trap is suitablydesigned for each individual engine design, because the particulateemissions differ significantly from engine to engine. The trap may, butneed not, carry a catalytic coating intended to initiate combustion atlower temperatures.

Continuing development of the inventive concept of the present inventionhas led to studies of flow and pressure patterns within such exhausttreatment systems. A further embodiment of the invention, in which thetrap has peripheral bypass through which the exhaust gases flow onlywhen the central portion of the trap becomes blocked, is particularlypreferred. The flow of the gases can be directed to the central portionby a variety of means, including particularly baffle plates or cones,metal lips and the like, but we have found that extending the catalystin the peripheral area is particularly simple and effective to reducegas flow rates in that region.

A preferred embodiment of the present invention combines the emissioncontrol system with NOx control means. The NOx control may be achievedin a number of ways including exhaust gas recirculation, using a NOxconversion catalyst downstream of the trap or, more preferably,combining the trap system with a NOx absorbent. Such NOx absorbents areknown to those skilled in the art and may utilise an alkaline earthmetal oxide such as baria or calcia or other suitable materials.Together with the trap system of the present invention, such anabsorbent can permit extremely useful control of emissions, for exampleup to about 80% removal of particulates combined with up to about 80%removal of NOx. The NOx trap is desirably a single through-flowcanister, which may be regenerated by periodic enrichment of the exhaustgas in a number of ways. In an even more preferred embodiment, the trapsystem of the invention is combined with a lean-NOx catalyst and a NOxtrap. A particularly desirable embodiment is where the NOx trap iseffective to trap the NOx at low temperatures and releases NOx at highertemperatures, eg about 250° C., at which temperatures NOx may morereadily be converted and/or used in the particulate combustion of thepresent invention.

When using the presently-preferred platinum-based catalyst, the presentinvention should be us ed with fuel of not more than 50 ppm sulphur, andpreferably below 50 ppm sulphur, more preferably below 10 ppm sulphur.Other catalyst systems may have a wider range of fuel sulphur levels.

The invention may be better appreciated by reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic cross-section of a first embodiment of theinvention,

FIG. 2 is a schematic cross-section of a second embodiment of theinvention, and

FIG. 3 is a schematic cross-section of a further embodiment of theinvention, and

FIG. 4 is a schematic cross-section of a yet further embodiment.

Referring to FIG. 1, a canister is to be mounted in the exhaust systemof a light duty diesel, eg a 1.9 litre Tdi engine. The canister, 1,contains an oxidation catalyst, 2, which is a platinum catalyst carriedon a 100 cells/sq in metal honeycomb substrate. Particulate passesthrough the catalyst. A perforated gas distributor, 3, is mounteddownstream of the catalyst, and surrounding the gas distributor is asintered stainless steel filter, 4, which is located centrally withinthe canister. It can be seen that it is possible for the exhaust gasesto by-pass the filter if the filter becomes clogged. Under idealconditions, the filter collects sooty particles which are continuouslycombusted in the NO₂ generated by the catalyst, according to one or moreof the equations:NO₂+C→NO+CO2NO₂+C→2NO+CO₂2NO₂+2C→N₂+2CO₂NO₂+O₂+C→2NO+CO₂

Under non-ideal conditions, that is at low exhaust gas temperatures, aportion only of the particulate is collected in the filter, and most ofthe exhaust gas by-passes the filter. Returning to higher exhaust gastemperatures permits the combustion reaction to re-start and theparticulate can be totally removed from the filter.

Referring to FIG. 2, an embodiment is shown which permits substantialaccumulation of particulate without by-pass, but using a filter, 5,which is not 100% effective. The same items as in FIG. 1 are identifiedusing the identical reference numerals. There is sufficient capacity toaccumulate particulate under all normal operating conditions.

FIG. 3 utilises a slightly different by-pass design to that of theembodiment of FIG. 1. The particulate is collected by impingement on abaffle plate, 6, and is shown by mass, 7. The baffle plate is itselfpreferably porous to gas and acts as a filter. As exhaust gastemperatures rise, the hot gases immediately contact the collectedparticulate and quickly cause combustion. This design may compriseelectrical heating of the collection area on plate 6, creating a hotspot to initiate combustion. In a further design variation the baffleplate may comprise upstanding walls, giving a U-shaped cross-section.

A preferred embodiment is illustrated in FIG. 4. Engine exhaust gaspasses through catalyst 2, which is provided with a peripheral lip 2 a.The resulting increased resistance to flow in the peripheral regioncauses the majority of the gases to flow through the central, filtering,region of trap 4, (4 a) rather than through the open peripheral bypassregion (4 b). Thus under normal operating conditions, negligiblequantities of the exhaust gases bypass the filter, but if the filterbecomes blocked with particulate, the system is fail-safe and the gasesbypass the filter. Surprisingly low back pressures result from thissystem.

The above FIG. 4 embodiment was tested fitted to a commerciallyavailable 1.9 litre Tdi car designed to meet European Stage IIstandards. After 1,000 road miles, the embodiment was tested accordingto the European Stage III test protocol. The following test results wereobtained:

HC CO NO_(x) PM (g/Km) (g/Km) (g/Km) (g/Km) Engine-out exhaust gas 0.210.69 0.65 0.10 With FIG. 4 by-pass filter 0.02 0.03 0.62 0.02

It is to be noted that these results meet Stag IV requirements also,with the exception of NO_(x). However, established enginedesign/management techniques can be used to lower NO_(x) emissions toStage IV levels, with a consequent increase in Particulate Matter, butthe system of the invention is capable of dealing with such emissions.

It will be appreciated that there are many possibilities to vary thedesigns shown herein without departing from the principles of thepresent invention.

1. An emission control exhaust gas aftertreatment apparatus for exhaustgases from diesel engines comprising a source of NO₂, a wall flowfilter, means for directing flow of gases to a central portion of thefilter, and an exhaust gas by-pass effective under all operatingconditions, wherein a portion of the exhaust gases do not pass throughthe filter, such that up to 85 wt % of engine-out particulates arecollected on the filter and combusted In the presence of said NO₂ in thefilter.
 2. An apparatus according to claim 1, wherein the source of NO₂is a catalyst which is effective to convert at least a portion of NO inexhaust gases to NO₂.
 3. An apparatus according to claim 2, wherein theexhaust gases pass through the catalyst before passing through thefilter.
 4. An apparatus according to claim 1 further comprising a NO_(x)control means.
 5. An apparatus according to claim 4, wherein said NO_(x)control means is a NO_(x) absorbent.
 6. An apparatus according to claim1, wherein the wall flow filter has peripheral regions that define theby-pass.
 7. An apparatus according to claim 1 further comprising acatalyst carried by the wall flow filter.
 8. An apparatus according toclaim 1, wherein the filter is effective to trap about 50 wt % or moreof engine-out particulates.
 9. A method of controlling emissions fromdiesel engine exhaust gases by trapping and subsequently combustingparticulate matter, the method comprising the steps of directing theflow of gases to a central portion of a particulate filter, trapping upto 85 wt % of particulate matter in the exhaust gas in the particulatefilter, combusting the trapped particulate matter in the presence ofNO₂, and causing a portion of said exhaust gases to by-pass theparticulate filter under all operating conditions.
 10. A methodaccording to claim 9, wherein the trapping step comprises trapping about50 wt % or more of particulate matter in the exhaust in the particulatefilter.
 11. An emission control exhaust gas aftertreatment apparatus forexhaust gases from light duty diesel engines comprising a source of NO₂,a wall flow filter, means for directing flow of gases to a centralportion of the filter, and an exhaust gas by-pass effective under alloperating conditions, wherein a portion of the exhaust gases do not passthrough the filter, such that up to 85 wt % of engine-out particulatesare collected on the filter and combusted in the presence of said NO₂ inthe filter.
 12. An apparatus according to claim 11, wherein the sourceof NO₂ is a catalyst which is effective to convert at least a portion ofthe NO in the exhaust gases to NO₂.
 13. An emission control exhaust gasaftertreatment apparatus according to claim 11, wherein about 50 wt % ormore of engine-out particulates are collected on the filter.